860 
constitutes the northern portion of the so-called 
Bantul graben, extending southward from Yogyakarta 
town (Rahardjo, Sukandarrumidi and Rosidi, loc. 
cit.; Van Bemmelen, 1949), where much of the lahar 
activity of the Merapi volcano must have been 
concentrated in the past in a zone of tectonic 
downwarping. 
4. RELATION G.MERAPI-MENOREH HILLS- K. PROGO. 
The fluviovolcanics of the Merapi meet with the 
andesite rocks of the Menoreh hills ( also referred 
to as the West Progo Mountains, van Bemmelen 1949) 
just south of the confluence of the K. Sileng and 
the K. Progo , the main stream of the area ( fig. 
1). 
The Merapi volcano produces material at a high 
rate; van Bemmelen (1949) estimates it at about 6.4 
million m3/year averaged over the last 120 years, 
and its notorious lahars have an excessive carrying 
capacity. 
In fig. 2, some long profiles are shown of river 
sections in the area of study; the steepness of the 
K.Pabelan and the K.Blongkeng, both deriving from 
the G. Merapi, with respect to the other rivers ,is 
illustrative. 
The growth of the Merapi mountain body has gone 
accompanied by slipfaulting towards (a.o.) the 
southwest ( van Bemmelen, 1949 ; Bahar, 1985), 
causing crumbling and folding of the mountainfront 
— 
Qsmo 
Qb 
MW 
2 
^ HP 
^ A 
Q. 
—1 
• Fc 
p 
Æ ld 
i LD2 
.11 
dp 
D 
JT 
Fig.5. Sketch map showing the setting north and 
northwest of the Borobudur plain 
of the ai 
upwarpin; 
K.Progo, 
This is 
of the 
fig. 3 
debouchu: 
Van Bern 
has been 
Lake’ , : 
The laki 
retracti 
ponded 
consider 
judged 
mainly c 
The are 
been co 
borne ou 
5. PRESE 
5.1 K.Si 
The pla 
drained 
Menoreh 
eastward 
south, 
(for La 
metres a 
East of 
lake pi 
assuming 
back to 
Finally 
in a g 
into the 
5.2 The 
Affluent 
absent ( 
northern 
flatfloo 
discerna 
désignât 
4, and 
drainage 
lake. 
If any w 
in thes 
anymore 
into th 
cultivât 
5.3 The 
The Kali 
of abou 
course 
levels 
phases o 
The ine 
Progo r 
the emp 
late pha 
Correspo